If you're not used to using the keys it can take a few years for your little finger to catch up in terms of dexterity and accuracy. It'll likely be hit or miss for quite a while.

I recently acquired my first keyed flute. I had always planned on a six-key upgrade from my keyless D flute, but an 8-key came along at the right time (a secondhand Tom Aebi, Rudall style).

I've come to terms with the upper 6 keys, but getting a good solid note on the lower C# and C is a struggle. It doesn't help that this flute has the historical pewter plugs, but I think most of it is just getting my embouchure drilled into shape for those keys. If I nail the embouchure perfectly for a low D, then I can hit the C# with the key. The Cnat doesn't sound at all. So that's progress, and it makes me think it's my embouchure more than the key mechanics or a leak somewhere (I've done the usual suck/blow tests). Either way, I think you're getting into a fussier "Irish" flute if you're aiming for a solid low C foot with keys.

@Jim Stone: If your repertoire -- in addition to Irish trad -- is Beatles tunes, Rock, Classical, show tunes etc., then why wouldn't you use a Boehm flute? Seems like a better path, instead of forcing a 19th Century keyed flute design into that repertoire?

If your low C and C# keys aren't working well you may want to have them looked at by someone who knows their stuff like Jon Cornia. They should work well. Sometimes, because they aren't used as much they get dirty or we can bang the mechanism off just a bit. Forgive me for hijacking the thread, but it seemed like a comment was warranted. A simple cleaning might do the trick. You are right the embouchure needs to catch up with the low notes, but they shouldn't be that hard to sound if your flute is working correctly.

After some further practice and checking in the mirror, I think y'all might be right about the R3 finger slipping as the main, or at least contributing culprit for the low C and C#. That's going to need some attention, thanks for the advice!